Camshaft-to-connecting rod clearance is critical with this long a stroke, requiring that the cam bores be align-honed and fitted with oversized bearings. The cam is machined from a special reduced-radius, 1-inch base circle lobe designed to clear the rods as they swing much further outward on the massively stroked crank. Valvetrain components include Comp Cams' R-series T-bar hydraulic roller lifters, super-duty valve springs, and 1.6-ratio roller rockers, a package that, in concert with the CXI cam, will resist valve float to the 421's 7,500-rpm redline. The cam specs out with .598-inch gross lift on both intake and exhaust, while the duration at .50-inch lift is 244 degrees on the intake and 246 degrees on the exhaust. Lobe separation is 114 degrees, befitting a big-inch engine intended for streetability and for kicking ass when the occasion arises. The billet steel connecting rods, 5.850-inch standard length Olivers, feature special undersized big ends and relocated rod bolts, to reduce the proximity of cam and rods, and are symmetrically machined for added clearance.
Each 421 small-block is assembled by Pete Klemm and Jay Schuster, using custom JE forged low-silicone aluminum pistons. These pistons utilize short skirts to clear the throw of the crank, feature reverse domes, and are set up for full-floating pins. The top rings are plasma coated, while the second rings are of standard, cast-iron design. The ring groove spacing is wider than stock for additional ring-land strength under heavy nitrous applications. In concert with the lightly milled cylinder heads, the compression ratio works out to a stout 12.0:1. Thanks to an artful GM engine management recalibration by Eclipse Engineering in Whittier, California, the 421 runs happily and without detonation on 93-octane pump gas.
The heads start off as LT4 castings, thoroughly worked over by Gallant Technical Performance for serious maximum airflow and velocity, which is essential to making maximum power across a broad powerband-in this case, from off-idle to 7,500 rpm. Maximum flow, as measured on the GTP flow bench, is 300 cfm on the intake side, and 220 cfm on the exhaust. GTP also hand-ports the LT4 intake manifold, which is first cut apart by CXI, from plenum to the head ports, and opens up the throttle body bores to match a dual-throat Arizona Speed and Marine 58mm throttle body, which flows 1,000 cfm. A set of Ford Racing Technology (formerly SVO) 30 lb-hr injectors and a high-volume fuel pump provide both reliability and 600-700 horsepower worth of fuel, and an MSD 6A ignition with nitrous timing retard handles the combustion requirements.
A McLeod dual-disc clutch and lightweight steel-faced aluminum flywheel provide a surprisingly light touch for a setup capable of reliably transferring 1,000-plus nitrous-assisted horses rearward through a beefed-up ZF six-speed. The C4's four-wheel independent suspension has been tweaked with Koni double-adjustable "yellow" shocks and Suspension Engineering urethane bushings. A six-point rollcage meets both NHRA safety requirements and adds a needed dose of extra stiffness to the Vette's chassis, which has been lowered an inch at the front and 11/4-inches aft. ZR-1 wheels and BFG Comp T/A R1s are used on the street, while drag radials struggle to apply some of the prodigious power during the CXI 421 C4's drag strip forays. Seriously needed braking capabilities are on hand, thanks to huge 13.5-inch Baer brakes with four-piston calipers planted at all four corners.
The reality of the car is that there's no longer a need to look for power, but how to deal with too much power. In other words, "What stock part breaks next?" CXI's Jay Schuster relates that he's broken both halfshafts, shattered the differential case, and twisted the Dana 44 housing sideways and open about half a foot. By now the CXI folks have learned exactly how far you can go with a stock-type C4 suspension under almost any conditions.